1. Field of the Invention
This invention relates to an electrochemical storage cell of the alkali metal-and-chalcogen type with at least two spaces (anode and cathode space) separated from each other by an alkali-ion-conducting solid electrolyte for receiving the reactants, with the cathode space containing as catholyte at least one chalcogen or chalcogenide in dissolved or melted form, preferably sulfur or sulfides, as well as carbon- or graphite felt as the electrode material.
2. Description of the Prior Art
German Published Non-Prosecuted Application No. 2 633 345 describes such a storage cell. The essential purpose of the felt in the German storage cell is to increase the boundary surface, required for the electrochemical reaction, between the sulfur or the alkali sulfide produced during the discharge in the cathode space, and the current collector on the cathode side. The felt can be considered as the extension collector. Also, the spacing between the solid electrolyte and the cathodic current collector extended in this manner becomes very much smaller, whereby the resistance of the catholyte, which is very high particularly in the case of the fully charged state (sulfur phase), contributes substantially less to the internal resistance of the cell. Through this measure, satisfactory discharging of the cell is possible also if initially the pure sulfur phase is present in the cathode phase. During the discharge, which is normally carried only to Na.sub.2 S.sub.3, the conductive sodium polysulfides then appear in the cathode space to an increasing degree.
If the cell is recharged, elemental sulfur is instantaneously formed on the surface of the electrode, i.e., of the carbon- or graphite felt in this case, especially in the vicinity of the solid electrolyte. Since sulfur does not conduct, it blocks to some extent the continuation of the recharging process. This becomes the more critical, the more the sodium content in the cathode space drops and the sulfur content increases. As long as the composition of the melt is between Na.sub.2 S.sub.3 and Na.sub.2 S.sub.5, one still has the single-phase region with good mass exchange and the elemental sulfur formed will react rapidly with the polysulfide melt and form a polysulfide containing more sulfur, i.e., Na.sub.2 S.sub.3 will become Na.sub.2 S.sub.5, taking on two sulfur atoms. If the charging cycle is continued, then the cathodic reactant becomes two-phase, the one phase consisting of elemental sulfur and the other of Na.sub.2 S.sub.5, which is saturated with sulfur.
Additional difficulties arising in this region have been recognized and attempts have been made to meet them, particularly by modifying the carbon- or graphite felt.
German Published Non-Prosecuted Application No. 2 633 345, attempted to overcome the difficulties by different porosity and different electric conductivity of the felt of graphite or carbon.
According to German Published Non-Prosecuted Application No. 2 622 404, a porous graphite felt is coated with material of polar or ionic character or with unfilled d-orbitals. For this purpose, metal oxides and sulfides, especially alpha-aluminum oxide, molybdenum disulfide, chromium trioxide, lanthanum chromite and tin oxide are proposed.
The compounds mentioned are to cause faster desorption of the sulfur produced during the recharging by spoiling the wetting of the sulfur on the graphite surface. In this manner, favorable effects in the direction of faster charging can be obtained at operating temperatures from 350.degree. C. on, where the sulfur vapor pressure is several hundred Torr. This is not the case if the cell is operated at lower temperatures, as is desired nowadays for several reasons (among others, less corrosion, cost advantages).